Third as well as fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems.
For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as video streaming and video conferencing on mobile communications devices that would previously only have been available via a fixed line data connection. The demand to deploy fourth generation networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to increase rapidly.
The anticipated widespread deployment of fourth generation networks has led to the parallel development of a class of communications devices and applications which, rather than taking advantage of the high data rates available, instead take advantage of the robust radio interface and increasing ubiquity of the coverage area. One example of such an application is public safety communications, for instance communications between members of the emergency services. Public safety communications require a high degree of robustness and therefore fourth generation networks provide cost effective solution to public safety communications compared to dedicated systems such as TETRA which are currently used throughout the World. However, for public safety applications it is highly desirable that public safety communications devices can still communicate with each other even when outside of a coverage area provided by an LTE system. In LTE release-12 the ability for LTE devices to perform device-to-device communications has been introduced. This therefore allows LTE device to communicate with each other when outside of a coverage area but within close proximity of one another. This device-to-device communications ability allows LTE public safety communications even when there is no network coverage. However, the use of LTE for both high-reliability public safety communications and commercial applications may lead to complex resource allocation.